The present invention relates in general to sewing machines and in particular to a new and useful thread cutting device for lockstitch sewing machines having a rotary hook rotating in a horizontal plane with a bobbin case mounted in the rotary hook, a thread catcher and a cutting tool cooperating with the thread catcher to cut a needle and bobbin thread and, at the same time hold the bobbin thread.
A thread cutting device for a sewing machine in disclosed in German Pat. No. 23 25 609. That device comprises a thread catcher which is pivotable in a horizontal plane above a rotary hook and is designed with a separating finger formed on an arm, and a catch shoulder provided below the finger. At the underside of the separating finger, close to a pointed end thereof, a hook for catching the bobbin thread is mounted, permitting a long bobbin thread end portion extending to the thread supply.
To effect a cutting operation, the thread catcher is moved upon the instant at which the needle thread loop is passed around about one half the bobbin case, in a first pivotal step, from an initial position into a catch position, with the separating finger penetrating into the needle thread loop below the loop at the supply side, but above the bobbin thread, and the catch hook taking hold of the bobbin thread. After this first pivotal step, the thread catcher stops until the needle thread loop is passed around the entire bobbin case and pulled upwardly, by the take-up lever on the sewing machine, up to the thread catcher. Only after the needle thread loop is engaged on the separating finger and the legs of the needle thread loop come into a firm, controlled position, the thread catcher is moved farther in a second pivotal step, in the direction of the stationary cutting knife.
At the start of the second pivotal step, the catch shoulder applies against the needle thread loop, whereby, in the course of this second pivotal step, the needle thread is pulled off the thread supply, in addition to pulling off the bobbin thread. Since during this operation, the needle thread loop is firmly engaged on the separating finger so that still another deflection point is present in addition to the almost rectangular deflection at the stitch hole edge, and the other deflection points by which the pulling of the needle thread off the thread supply is anyway braked, namely the needle eye, the take-up lever, the thread tightener, and the various eyelets for the thread, a relatively strong force is required for performing the second pivotal step of the thread catcher, even with the tightener in an open position. With thick threads having unfavorable frictional properties, it may even be necessary to equip the thread cutting device with an auxiliary drive, agumenting the driving force.